The invention is directed to a web pick-up device and method of using the device in the press section of a machine for making fibrous webs of paper, cardboard or the like. Web pick-up devices in the press section of a machine for making fibrous webs are known in the art. Running webs make contact in the press nip with a smooth rotating surface from which the web runs off at the point of pickup. The rotating surface and a suction box form together, in the area of the pick-up point, a gap through which runs a porous conveyor belt which receives the fibrous web.
Web pick-up devices of this general type are known, for example, in the following publications:
1U.S. Pat. No 4,016,032 PA0 2. Ep-A- 0364114, PA0 3. EP-A- 0344088 (U.S. Pat No. 4,943,351) PA0 4. Ep-A- 0276202. PA0 5. AT-PS 372,429 PA0 6. U.S. Pat. No. 2,780,968.
Similar devices are known from:
On the devices known from publications 1 and 2, a suction roll is arranged on a press roll which features a smooth roll shell that makes direct contact with the fibrous web. A porous conveyer belt, preferably a so-called dry wire, runs across this suction roll. The press roll and the suction roll form together a gap through which passes the conveyer belt. The objective with this arrangement is to have the porous conveyer belt (by means of the suction prevailing on the suction roll) pick up the fibrous web from the press roll and transfer it to the subsequent drying section. This is achieved so that the pickup of the web from the smooth press roll occurs in a way such that the web will constantly be supported by some means of transport, namely first by the shell of the press roll and thereafter by the conveyer belt.
This is to avoid the presence of a free web train between the press shell and the conveyer belt. As is generally known, such a free web train causes an undesirable longitudinal stretch in the fibrous web, which at this point is still moist and possesses only little tensile strength. It must also be taken into account here that the still moist fibrous web clings at a relatively high adhesive force to the smooth shell of the press roll. By selecting a suitable shell material (for instance granite, artificial stone or plastic) it is attempted to reduce the adhesive force. Nevertheless, the longitudinal stretch of the web at the point of pickup remains a problem. Due to this longitudinal stretch, it happens relatively frequently during the operation that the fibrous web breaks at the point of pickup or in the subsequent drying section, thereby interrupting the production.
It had been difficult to establish the use of the aforementioned devices (where an attempt is made to avoid a free web train) in practice because of other difficulties. One of these difficulties consists in the following: As known, modern paper machines are generally dimensioned for an extremely large web width (in the order of up to 10 m). Therefore, all of the rollers must have a correspondingly large length, for which reason they undergo, under their deadweight and the respective load, a more or less heavy sagging. In the case of the known web pick-up devices, the consequence of this is that it is practically impossible to produce an exactly uniform gap between the press roll and the suction roll, without forcing the suction roll on the press roll. But such contact pressure must be avoided because the web would otherwise be damaged by the porous conveyor beIt It must be taken into account here that the porous conveyor belt (preferably a so-called dry wire, which subsequently carries the web from the point of pickup through part of the drying section) has a considerably coarser structure than the wet felts used in the press section, and that the conveyor belt (normally) must be made endless by means of a seam.
Further known web pick-up devices have been described in publication 3. According to FIG. 17, the fibrous web to be dewatered runs between a felt and a smooth surface of a press belt through the press gap. Behind the press gap, the press belt and the web run with the roll shell and the press roll up to a point of pickup where a conveyer belt (for example a dry wire) is tangent with the press roll shell and receives the web from the press belt. Behind the point of departure, the conveyer belt runs together with the web along a straight section to a roll situated inside the conveyer belt loop (guide roll or drying cylinder).
In other embodiments of the publication 3, the press belt runs together with the web from the press gap first across an additional support roll, on the circumference of which the point of pickup is provided; that is, the dry wire touches at the point of pickup the support roll, picking the web up at that point; in several embodiments again with the aid of a vacuum box which, for example, may be fashioned as a suction box. All of the devices known from publication 3 have the disadvantages that the mentioned press belt means not only additional expenditure, but may now and then also cause an interruption of the operation, namely when it needs to be replaced by a new press belt because of wear.
In the case of another known web pick-up device (publication 4), the fibrous web to be dewatered runs directly on the smooth surface of the press roll shell from the press gap to the point of pickup, where it is received again by a conveyer belt. For separating the web from the smooth surface of the press roll there is an electrical induction heater provided. Interacting with the magnetically conductive shell material of the press roll, this heating device is supposed to effect a noncontact heating of the roll shell, and specifically the fibrous web, in order to thereby reduce the adhesive force of the moist web on the press roll shell. However, such a device involves a high consumption of energy. Besides, as compared to a simple suction device, it would appear to be more difficult to establish a reliable and troublefree continuous operation.
The problem underlying the invention is to provide a web pick-up device, wherein during continuous operation the pickup of the fibrous web from the smooth press roll and the advance of the web to the following drying section can take place with greater safety than heretofore, i.e., with a lessened risk of web breaks. At the same time, just as with the known arrangement, a free web section is to be avoided, so that in the region of the web pick-up device no longitudinal stretch of the fibrous web will occur, or at the most a very slight longitudinal stretch will occur.